Major histocompatibility complex (MHC) class I molecules present antigens to the adaptive immune system. The class II transactivator, CIITA, is the limiting factor and master regulator of MHC-II gene expression. Despite considerable analysis of CIITA function, the molecular mechanisms controlling its regulation and therefore a cell's ability to present antigens are not fully understood. Until recently it was thought that three major promoters and proximal regulatory regions, which functioned in a cell type dependent manner, controlled CIITA. We now know that there are multiple regulatory elements at long distances from each of the promoters. How these elements control cell type specific promoter usage, share key regulatory elements and factors, and interact with each other and chromatin insulators is not known. No detailed information on how CIITA is regulated in dendritic cells is known. This is surprising as these cells provide the initial contact with pathogens and initiate MHC-II-mediated immune responses. Aim 1 will address these unknowns and develop a comprehensive model on how CIITA is regulated. The B cell to plasma cell transition represents a major cell fate decision and requires extensive genetic and epigenetic reprogramming of the cell. During this transition CIITA expression is silenced. This silencing was thought to require only the presence of the plasma cell fate repressor Blimp-1. We provide compelling evidence that the transcriptional repressor Zbtb32 is required for CIITA silencing and likely plays a much broader role in plasma cell differentiation. Blimp-1 binding to the CIITA gene required Zbtb32. Zbtb32 can associate with Ezh2, a histone methyltransferase associated with epigenetic gene silencing mechanisms and cell fate decisions. Aims 2 and 3 will identify Zbtb32's mechanism of action, breadth of genes it regulates, determine its role in plasma cell differentiation, and lastly whether Zbtb32-deficient animals develop a humoral immune response to infection with H1N1 influenza and LCMV. This work will define the molecular characteristics that regulate CIITA and MHC-II gene expression, and characterize a unique pathway for MHC-II silencing and plasma cell fate decisions. Data generated here will have a broad impact on our understanding of how adaptive immune responses are controlled and should lead to new insights into our ability to design novel therapeutics for transplantation and to treat infection, cancer, and autoimmunity.